Quantum chemical study of the formation of acetylenic alcohols and 7-methylidene-6,8-dioxabicyclo[3.2.1]octanes from acetylene and carbonyl compounds

The geometric structures of the conformers of 2,3,4-trimethyl-7-methylidene-1,5-di-(thiophen-2-yl)-6,8-dioxabicyclo[3.2.1]octane were studied by the MP2/6-311++G**// B3LYP/6-31G* method. The average deviations of the calculated bond lengths from those determined by X-ray diffraction are no larger than 0.02 A, and the deviations of the bond angles and dihedral angles are 0.7 and 3.5°, respectively. In the gas phase, the chair conformation is thermodynamically ∼7.3 kcal mol−1 more favorable than the boat conformation, and, taking into account the solvent effect of DMSO, the chair conformation is ∼9.4 kcal mol−1 more favorable. For formaldehyde and a series of ketones, including alkyl hetaryl ketones, the enthalpies of competitive formation of acetylenic alcohols and 7-methylidene-6,8-dioxabicyclo[3.2.1]octanes were calculated. The formation of the latter compounds is characterized by a considerable decrease in the enthalpy (to −91 kcal mol−1).